1. Field of the Invention
The present invention relates to a substrate for liquid crystal display elements, and more particularly, to a substrate for liquid crystal elements for use in semi-transparent type liquid crystal display elements which partially transmit light.
2. Prior Art
In recent years, in order to meet demand for thinner and more lightweight portable electronic devices and apparatuses as well as requirements for a longer battery driving time, reflective-type liquid crystal display elements utilizing external light have become used to realize low electric power consumption liquid crystal displays. However, since the display quality (especially in terms of contrast) of such reflective-type liquid crystal display elements rely heavily on external light, it is not possible to obtain sufficient brightness in a dark place compared with transparent-type liquid crystal display elements which utilize back light, leading to a degraded picture quality. One method to overcome this problem has been proposed by Japanese Laid-Open Patent Publication (Kokai) No. 11-002709 which provides a semi-transparent type liquid crystal display element using a semi-transparent plate as a reflector so that the element may be used both as a reflective type in a bright place and a transparent type using back light in a dark place.
A substrate for liquid crystal display elements used in this semi-transparent type liquid crystal display element has a liquid crystal layer interposed between a pair of transparent substrates provided with liquid crystal-driving electrodes, such that a light scattering characteristic of the liquid crystal layer is controlled by the magnitude of voltage applied to the liquid crystal layer. Further, this substrate has such a structure that a semi-transparent reflector formed of metal, such as aluminum, is stacked on a substrate on a backside, wherein the semi-transparent reflector is formed of a metallic thin film which has a thickness reduced to such a degree that light can partially be transmitted through the film.
However, to obtain such a metallic thin film with a reduced thickness, it is necessary to make the film thickness extremely thin to suppress the reflection of light and hence increase the degree of light transmittance. Therefore, a high degree of control of the thickness of the film is required during the manufacturing process, thus making it difficult to realize such a thin metallic film. Moreover, even if the film thickness is reduced, light is absorbed during transmission of light so that the utilization factor of light is low.
Further, when the liquid crystal is driven, capacitance occurs between the metallic thin film and transparent electrodes (transparent conductive films), which induces a signal delay, and thus there is a possibility that the speed of a drive signal for driving the liquid crystal display element lowers.
It is therefore an object of the present invention to provide a substrate for liquid crystal display elements which can meet a variety of required optical characteristics and, at the same time, can improve the utilization factor of light without the possibility of inducing a signal delay.
To attain the above object, the present invention provides a substrate for liquid crystal display elements comprising a transparent substrate, and a predetermined number of pairs of a first transparent film having a high refractive index and a second transparent film having a low refractive index, each composed of a dielectric material and stacked on the transparent substrate, wherein: the first transparent film has a refractive index of light of not less than 1.8 at a wavelength of 550 nm, and the second transparent film is stacked on the first transparent film and has a refractive index of light of not more than 1.5 at the wavelength of 550 nm; the predetermined number is an integer not less than 1; and the first transparent film and the second transparent film each have a film thickness thereof set to such a value that the light reflectance in a visible light region of each of the first and second transparent films is within a range of 5-95%.
According to the substrate of the present invention, it is possible to meet a variety of required optical characteristics, for example, it is possible to freely set the ratio between light transmittance and light reflectance in the visible light region of each transparent film over a wide range according to applications, and in addition, it is possible to improve the utilization factor of light since a transparent film having a high refractive index and a transparent film having a low refractive index, each composed of a dielectric material, are used. Further, since neither of the transparent films is composed of a metallic thin film, the possibility of inducing a signal delay can be eliminated.
Preferably, the substrate for liquid crystal display elements includes a transparent roughened surface scattering layer stacked on the transparent substrate.
As a result, glare by reflection of light can be suppressed.
It is preferable that the light reflectance in the visible light region of each of the first and second transparent films is in a range of not less than 5% but less than 25%, wherein: when the predetermined number is 1, the first transparent film has a film thickness of 20-130 nm, and the second transparent film has a film thickness of 50-110 nm; when the predetermined number is 2, the first transparent film has a film thickness of 5-60 nm, and the second transparent film has a film thickness of 5-150 nm; when the predetermined number is 3, the first transparent film has a film thickness of 3-80 nm, and the second transparent film has a film thickness of 5-160 nm; and when the predetermined number is 4, the first transparent film has a film thickness of 5-80 nm, and the second transparent film has a film thickness of 5-80 nm.
It is also preferable that the light reflectance in the visible light region of each of the first and second transparent films is in a range of not less than 25% but less than 45%, wherein: when the predetermined number is 1, the first transparent film has a film thickness of 80-110 nm, and the second transparent film has a film thickness of 40-60 nm; when the predetermined number is 2, the first transparent film has a film thickness of 20-180 nm, and the second transparent film has a film thickness of 30-100 nm; when the predetermined number is 3, the first transparent film has a film thickness of 10-130 nm, and the second transparent film has a film thickness of 10-170 nm; when the predetermined number is 4, the first transparent film has a film thickness of 20-110 nm, and the second transparent film has a film thickness of 5-100 nm; when the predetermined number is 5, the first transparent film has a film thickness of 10-110 nm, and the second transparent film has a film thickness of 5-110 nm; and when the predetermined number is 6, the first transparent film has a film thickness of 10-80 nm, and the second transparent film has a film thickness of 30-100 nm.
Further, it is preferable that the light reflectance in the visible light region of each of the first and second transparent films is in a range of not less than 45% but less than 65%, wherein: when the predetermined number is 2, the first transparent film has a film thickness of 60-180 nm, and the second transparent film has a film thickness of 40-90 nm; when the predetermined number is 3, the first transparent film has a film thickness of 20-160 nm, and the second transparent film has a film thickness of 10-150 nm; when the predetermined number is 4, the first transparent film has a film thickness of 20-180 nm, and the second transparent film has a film thickness of 10-110 nm; when the predetermined number is 5, the first transparent film has a film thickness of 30-190 nm, and the second transparent film has a film thickness of 10-140 nm; when the predetermined number is 6, the first transparent film has a film thickness of 10-150 nm, and the second transparent film has a film thickness of 10-100 nm; when the predetermined number is 7, the first transparent film has a film thickness of 20-150 nm, and the second transparent film has a film thickness of 5-110 nm; when the predetermined number is 8, the first transparent film has a film thickness of 20-130 nm, and the second transparent film has a film thickness of 5-110 nm; and when the predetermined number is 9, the first transparent film has a film thickness of 20-120 nm, and the second transparent film has a film thickness of 10-90 nm.
It is also preferable that the light reflectance in the visible light region of each of the first and second transparent films is in a range of not less than 65% but less than 95%, wherein: when the predetermined number is 3, the first transparent film has a film thickness of 80-160 nm, and the second transparent film has a film thickness of 40-110 nm; when the predetermined number is 4, the first transparent film has a film thickness of 60-140 nm, and the second transparent film has a film thickness of 40-100 nm; when the predetermined number is 5, the first transparent film has a film thickness of 30-130 nm, and the second transparent film has a film thickness of 20-170 nm; when the predetermined number is 6, the first transparent film has a film thickness of 20-180 nm, and the second transparent film has a film thickness of 10-140 nm; when the predetermined number is 7, the first transparent film has a film thickness of 10-150 nm, and the second transparent film has a film thickness of 30-130 nm; when the predetermined number is 8, the first transparent film has a film thickness of 5-200 nm, and the second transparent film has a film thickness of 5-150 nm; and when the predetermined number is 9, the first transparent film has a film thickness of 5-200 nm, and the second transparent film has a film thickness of 5-140 nm.
Since the thickness of each of the transparent film having a high refractive index and the transparent film having a low refractive index can thus be set over a predetermined range, the light reflectance in the visible light region can be set over a wide range, making it possible to meet a variety of required optical characteristics.
Preferably, the second transparent film is formed of a material having a low refractive index consisting essentially of at least one compound selected from the group consisting of silicon dioxide, magnesium fluoride, calcium fluoride, and lithium fluoride.
Silicon dioxide is particularly preferable because of its excellent chemical durability.
In a preferred embodiment of the present invention, the second transparent film includes a transparent film located farthest from the transparent substrate, the transparent film being formed of silicon dioxide and having a film thickness of not less than 20 nm.
As a result, it is possible to improve the degree of adhesion of the low refractive index transparent film which is located farthest from the transparent substrate, to other materials such as a color filter coated on the transparent thin film.
In a further preferred embodiment of the present invention, the first transparent film is formed of a material having a high refractive index consisting essentially of at least one compound selected from the group consisting of titanium dioxide, zirconium dioxide, tantalum pentoxide, and tin oxide.
The above and other objects, features and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.